1. Field of the Invention
The present invention relates to implementing high-impedance integrated circuit elements and more particularly to such elements having resistances of at least one megohm.
2. Description of the Related Art
In its simplest form, a low-pass filter comprises a resistor having first and second sides with the second side thereof being connected to ground via a capacitor. A signal is low-pass filtered by applying the same to the first side of the resistor. The filter output, i.e., the filtered signal, appears on the second side of the resistor. The time constant of the filter circuit, which is equal to the resistance of the resistor multiplied by the capacitance of the capacitor, determines which frequency components are passed by the filter and which are not. The greater the time constant, the lower the frequencies which are passed. Thus, when a filter which passes only very low frequencies is required, the filter time constant must be very large. A very large resistance or a very large capacitance must therefore be provided.
Using conventional integrated circuit designs for capacitors or resistors requires the use of large amounts of space in the circuit to provide either a large capacitor or a large resistor. For implementing a low-pass filter for a cut-off frequency of less than about 50 kilohertz, in order to use a conventionally-sized capacitor, the resistance must be in the one megohm range. A one megohm resistor implemented with conventional integrated circuit techniques would take up a tremendous amount of space.
Such filter circuits typically require resistances in the one megohm to one hundred megohm range. In many applications, e.g., certain low-pass filter applications, precise values of resistances are not necessary. So long as the resistance is sufficiently large, e.g., one megohm, circuit performance is satisfactory even if the resistance should be much larger, e.g., one hundred megohms, so long as it is not so large as to be, in effect, an open circuit. Some such applications for low-pass filters include, e.g., automatic gain control (AGC) circuits, DC control amplifiers, and other biasing circuits. In a similar way the resistive and capacitive elements can be interchanged to form a high-pass filter.